Question 40797
When an object goes upstream it needs more effort. If you do not give it effort it loses speed or goes back. We use subtraction. When an object goes downstream it moves fast and speed increases so we use addition.

Upstream: (r-3)
Downstream:(r+3)

upstream:(r-3)/9
downstream: (r+3)/11

What we must do now is cross multiply:

We have
(r-3)/9=(r+3)/11

When we cross multiply we now have:
11(r-3)= 9(r+3)

Distribute:

11r-33=9r+27

2r=60

r=30